docs: def resolve_model(self, model):
          '''
          Resolve a model given a name or dict with `class` entry.
  
          :raises ValueError: model specification is wrong or does not exists
          '''
          if not model:
              raise ValueError('Unsupported model specifications')
          if isinstance(model, basestring):
              classname = model
          elif isinstance(model, dict) and 'class' in model:
              classname = model['class']
          else:
              raise ValueError('Unsupported model specifications')
  
          try:
              return get_document(classname)
          except self.NotRegistered:
              message = 'Model "{0}" does not exist'.format(classname)
              raise ValueError(message)